Final answer:
Parathyroid Hormone (PTH) is secreted by the parathyroid glands; it regulates calcium balances by acting on bones, kidneys, and the digestive system, thereby increasing blood calcium levels.
Step-by-step explanation:
Parathyroid Hormone (PTH) is secreted by the parathyroid glands (option A). Its primary role is to regulate the body's calcium levels. PTH acts on several target tissues, including the bones, kidneys, and digestive system, with the effect of increasing blood calcium levels. In the bones, PTH stimulates osteoclasts, leading to the release of calcium into the bloodstream. In the kidneys, it enhances the reabsorption of calcium, thereby reducing its excretion. Lastly, the hormone stimulates the production of calcitriol which increases the absorption of calcium in the digestive system.